cli: move `revsets.log` default to config file

cli/src/config/revsets.toml

···
 # NOTE: ensure you update docs/revsets.md with documentation when
 # adding/updating any of these aliases
 
+[revsets]
+log = "@ | ancestors(immutable_heads().., 2) | trunk()"
+
 [revset-aliases]
 'trunk()' = '''
 latest(

lib/src/settings.rs

···
     }
 
     pub fn default_revset(&self) -> String {
-        self.config
-            .get_string("revsets.log")
-            .unwrap_or_else(|_| "@ | ancestors(immutable_heads().., 2) | trunk()".to_string())
+        self.config.get_string("revsets.log").unwrap_or_default()
     }
 
     pub fn signature(&self) -> Signature {